The U.S. space program has relied on the reusable space transportation system (space shuttle) to do the bulk of its work in carrying cargo and satellites into low earth orbit, as well as for retrieving satellites from orbit and performing maintenance and repairs on them. In general, this system has performed remarkably well, with each shuttle vehicle being reused many times. However, the recent shuttle disaster in 2003, where the shuttle Columbia broke up on reentry into the atmosphere due to a loss of integrity in the thermal protection tiles, brings to light the fallibility of the thermal protection tile system and raises a need for a means to guard against future disasters of this type. To date, there is no means for monitoring the integrity of the thermal tiles on the space shuttle during flight. While it has been attempted to assess the integrity of the tiles visually, by means of cameras or even by direct visual inspection during spacewalks, these methods are not very accurate as the tiles may be loose or cracked and these faults would not be observed visually.
U.S. Pat. No. 3,596,269 utilizes a number of resistance elements connected in parallel to indicate structural fatigue cracking and tearing in aircraft. Resistance elements are in parallel and apparently measure the degree of stress failure by determining the intensity of the resistance. One end of the circuit is grounded, and a complete break in the circuit would render this method useless.
U.S. Pat. No. 3,383,863 detects leaks in retaining pond linings by a grid of wires laid beneath the lining. When water or other fluid leaks from the pond through the lining an electrical circuit is completed between the separated intersecting connectors of the grid. The location of the leak is determined by measuring the resistance between crossing grid members, the leak being in the vicinity of the intersection of least resistance.
U.S. Pat. No. 4,503,710 describes a means for monitoring the integrity of a structure in a fluid tight environment, and measures a change in resistance due to a leak in the environment, which indicates a crack or break in the structure.
U.S. Pat. No. 3,477,019 shows an electrical circuit utilizing resistors to pinpoint the location of ground movement. The resistance is measured and a break in the circuit registers as a change in resistance, the magnitude of the resistance indicating the location of the break.
While the aforementioned patents indicate the use of resistance measuring circuits to detect the loss of integrity in the circuit, none specifically addresses the unique problems encountered in the thermal protection tiles of the space shuttle.